What happens to a positive back pressure EGR valve when vacuum is applied with the engine off?

In a positive back pressure EGR (Exhaust Gas Recirculation) valve system, the functioning of the valve relies on the engine's operating conditions. When vacuum is applied to the EGR valve while the engine is off, the valve does not open unless a specific condition is met, such as an obstruction or restriction that would create the necessary pressure differential.

This is because the positive back pressure EGR valve is designed to control the recirculation of exhaust gases based on the pressure dynamics related to the engine's operation. In the absence of combustion gases flowing through the exhaust system (as is the case when the engine is not running), there is no back pressure to push the valve open. Therefore, simply applying vacuum alone is insufficient to open the valve due to the design mechanism, which typically requires the appropriate back pressure to allow for operation.

Understanding this principle is crucial for diagnosing and troubleshooting EGR systems effectively, as it highlights the importance of engine operation and exhaust pressure in EGR functionality.